which command doesn't work

[Cesar Romani <cesar.romani@xxxxxxxxx>]

I've installed zsh 4.3.10 on the MSYS environment. It works fine but not
the which command.
I have three different versions of the "less" command installed:

$which -a less
/usr/local/bin/less
/bin/less
/c/Programmi/GnuWin32/bin/less

but
$which less
/c/Programmi/GnuWin32/bin/less

The which command takes the last version of less, although that version
is located at the end in my PATH. And when I want to use less, the shell
takes the last version of it. Why?

If I do
$/bin/which less
I get
/usr/local/bin/less.exe

I get the correct version of less, namely the first in my PATH.

Many thanks in advance,
Cesar